A semiconductor memory device is a memory device that stores data and enables the stored data to be read when necessary. Semiconductor memory devices can largely be divided into a volatile memory device and a nonvolatile memory device.
The volatile memory device is one in which stored data are erased when a power supply source is shut off. Volatile memory devices include static random access memory (SRAM), dynamic random access memory (DRAM) and synchronous dynamic random access memory (SDRAM). The nonvolatile memory device is one in which stored data are not erased even when a power supply source is shut off. Nonvolatile memory devices include Read Only Memory (ROM), Programmable Read Only Memory (PROM), Erasable Programmable Read Only Memory (EPROM), Electrical Erasable Programmable Read Only Memory (EEPROM), flash memory devices, Phase-change Random Access Memory (PRAM), Magnetoresistive Random Access Memory (MRAM), Resistive Random Access Memory (RRAM) and Ferroelectric Random Access Memory (FRAM). Flash memory devices can largely be classified into NOR type and NAND type devices. The NAND flash memory device provides a higher degree of integration than the NOR flash memory device, while the NOR flash memory device provides a higher operating speed than the NAND flash memory device.